The use of lasers in manufacturing, science, medicine, communication, and entertainment has grown rapidly throughout the industrialized world. For many of the laser-based technologies, an optical scanner is required to deflect the beam in a controlled fashion to follow a defined path. One commonly used type of scanning application is raster scanning, in which a deflection device moves a laser beam at constant velocity during an active portion of the scan cycle, then quickly returns the beam to the starting point to begin the next scan cycle. FIG. 1 shows a conventional sawtooth function used as a raster scanning system, where the x axis represents time and the y axis represents displacement. As shown in FIG. 1, the raster scan includes an active scan time t.sub.ACT, a return scan time t.sub.RET, and a cycle time t.sub.CYC. The ratio of the active scan time t.sub.ACT and the cycle time t.sub.CYC is called the duty cycle. High speed and high-duty cycle sawtooth scanning is desirable to meet the needs of many emerging technologies such as laser projection high-definition television and digital projection systems for movie theaters.
For raster scanning, the scan frequency translates to the number of frames per second which are projected onto the screen. The higher the number of frames per second, the better the quality of the observed image due to reduced flicker. Standard television images operate at 30 frames per second, while 60 to 90 frames per second are required for high definition television (HDTV). The motion picture industry uses 24 frames per second, but for digital projection systems 72 to 96 frames per second would be required.
There are at least six types of laser scanning technologies in use; resonant, electro-optic, acousto-optic, polygon, holographic, and galvanometric. However, each of these technologies has limitations which prevent their use in many of the emerging applications. For example, the resonant scanner can provide a line scan at high frequency and stability, but the scanned beam moves in a sinusoidal fashion, providing a nonlinear spot velocity and low duty cycle. Electro-optic and acousto-optic scanners can provide very high duty cycles and scan rates, but only small amplitudes are available. In addition, electro-optic and acousto-optic scanners suffer from a significant loss of laser power through the device. Multifaceted polygon scanners can achieve high scanning speeds, but they suffer from beam positioning errors due to facet-to-facet errors and a low-duty cycle for large aperture beams. Multifaceted holographic scanners can be designed to overcome the beam positioning errors of the polygon scanners, but they still provide low duty cycle for large aperture beams and suffer from loss of laser power through the device.
Galvanometric scanning technologies provide the best solution. A galvanometric scanner (galvo) is a limited rotation DC torque motor with an integral position sensor. Beam deflection optics, such as a mirror, are attached to a motor shaft, which can be moved under servo control to direct a laser beam to follow a command signal. The galvo combines the advantages of a single mirror surface, which gives consistent light reflectivity, low cycle-to-cycle error, and small package size.
While galvo beam steering technology has the capability of achieving high duty cycle raster scanning, conventional galvos are inadequate for the high speed and high-duty cycle sawtooth scanning required for many new technologies.
There are three conventional types of galvo motor designs used in the industry: moving coil, moving iron, and moving magnet. A moving coil design is based upon the d'Arsonval galvanometer design where the coil is supported by aluminum struts attached to a center shaft which runs the length of the rotor. A biasing magnetic field is produced around the rotor coil. When current flows through the coil, the rotor generates a torque proportional to the current. This type of scanner can generate high torque, but heat dissipation from the coil limits their performance. Also a flexible wire connection must be provided to the moving coil which can limit its long term reliability.
A moving iron motor has a stator which has stationary coils with biasing permanent magnets. The magnets produce a magnetic flux across the gap between the stator and a soft iron rotor. When the current flows through the stator coil, a control flux is created in the same air gap. This control flux interacts with the biasing flux to generate the drive torque in the rotor. Moving iron galvo motors have a simple construction and good heat conduction path from the coil to the heat sink. However, the gap between the rotor and stator has to be very small, less than 0.005 inch, to generate high torque. Further, the torque output is very sensitive to the gap, which results in large variation in motor torque due to manufacturing tolerances.
Currently available galvo systems use the moving magnet design. FIG. 2 shows a cross sectional view of a moving magnet motor 10 having a conventional two pole design. The moving magnet motor 10 includes two magnets 12 and 14 positioned on the rotor 16 to provide the two pole design. A control coil 18 is cemented on the inside of a soft iron cylinder 20 connected to the motor case (not shown). The control coil 18 surrounds the rotor 16. Conventional galvo systems use linear servo controllers to drive the moving magnet motor 10.
Moving magnet motor 10 is a conventional inside-out d'Arsonval design that works with larger air gaps. Consequently, motor 10 is not sensitive to manufacturing tolerances. While the heat dissipation and torque constant of the moving magnet design is not as good as that found in the moving iron design, the inductance is lower in the moving magnet design, which permits faster rise of current in the coil and, thus, higher acceleration capability.
For a conventional two pole moving magnet motor, such as that shown in FIG. 2, the torque output from the motor is: EQU T=torque (Newton meters)=B.sub.g LIN.sub.s D.sub.r eq. 1
where:
B.sub.g =air gap flux density (Tesla); PA1 L rotor length (meter); PA1 N.sub.s =Number of coil turns; PA1 I =Coil current (amps); and PA1 D.sub.r =Rotor diameter (meter).
Conventional galvo scanners use a capacitive position sensor attached at one end of the rotor shaft. The torsional stiffness between the load, i.e., the mirror, and the position sensor can cause unwanted structural resonances that result in jitter in the scanned beam. The jitter causes a loss of resolution in the system along with increased image distortion. Loss of resolution and increased image distortion are particularly unacceptable in laser based video projection systems.
Thus, there is a need for a galvo system that provides high-duty cycle sawtooth scanning at high scan rates, and large scan angle to meet the demands of the emerging laser based technologies and products.